If it rains you cant work outside with clay, it is against all odds. You can build anything you want in 6 hours, but depending on the thickness it will crack if you let it dry too fast, that is if you are using regular clay, If you are going to use air dry clay that might be a little bit different and might withstand the fast dry but this kind of clay cant be formed into too large pieces. For this kind of situation you might want to re-think the medium, maybe working with rocks, or branches, or any other material that wont become mud if it gets too wet or just fall apart because it is really brittle to be outside. If you give more information as for what is exactly what are you doing I'm sure you might get better responses.